Sound absorbing construction



SOUND ABSORING CONSTRUCTION Filed July 30, 1932 Patented Mar. 12, 1935 UNITED STATES SOUND ABSORBING CONSTRUCTION Kemper Slidell, Madison, Wis., assignor to C. F. Burgess Laboratories, Inc., Madison, Wis., a corporation of Delaware Application July 30, 1932, Serial No. 626,417

16 Claims.

This invention relates to an improved sound absorbing construction for the surfaces of walls, ceilings and the like.

It is an object of the invention to provide a unitary sound absorbing construction for walls, ceilings, and the like which has a high efllciency throughout both the low and high frequency ranges. Another object is to provide a unitary sound absorbing construction in which the relation between the absorption in both the high and lower frequency ranges may be controlled.

Inthe accompanying drawing, in which several embodiments of the invention are illustrated:

Fig. 1 shows one arrangement in cross section, somewhat schematically; and

Figs. 2, 3 and 4 show, in cross section, specific examples of the application of the invention.

Sound absorbing materials used on wall or ceiling surfaces and the like are made of a wide variety of materials, each of which has its sound absorbing characteristics. Examples of such absorbers are hair felt, mineral wool, rock wool, asbestos fibers, wool, cotton, excelsior, wood fibers, acoustic plasters, loose aggregates and porous ceramic products. These absorbers have the property of being most eflicient in the high frequency range, that is 500 double vibrations or cycles per second, and above, although their highest eiiiciency occurs in a limited range or band of frequencies. For example, mineral wool and hair felt have their highest efficiency at 500 to v 2000 cycles while an acoustic plaster has its highest efliciency above 2000 cycles. All of the above mentioned porous absorbers have a poor efficiency in the low frequency range, that is, below 500 cycles and particularly at 250 cycles and below. Even those which have a high efficiency at 500 cycles drop off rapidly as the frequency drops below that figure and have a low efliciency at 125 cycles.

The range of about 125 to between 4000.and 8000 cycles covers most of the frequencies encountered in acoustical correction work and it is with these frequencies that my invention is especountered in architectural acoustics including.

sounds based on vibrations of less than 500 cycles. In general, the outer facing consists of an abcially concerned. When low'frequencies are resorber or construction which has its highest efficiency in the high frequency range, that is, 500 cycles and above. This is backed by an absorbing construction which is most eflicient in the frequencies below the high efficiency range of 5 the outer facing and often at 125 to 250 cycles. Channels run from the face of the high frequency absorber to the low frequency absorber. It is necessary to have relatively few of such channels as hereinafter explained, to transmit low frequency sounds and obtain a high efficiency. On the other hand, if the low frequency absorber containing but a few openings is used for the exposed face, the high frequency sounds incident to it are reflected and remain largely unabsorbed.

A construction based on the principles involved in my invention is shown schematically inFig. 1 in which 10 is the wall or ceiling surface and the like to which the sounds to beabsorbed are incident. The compound absorber 11 of my invention is mounted on this surface by any suitable means as by gluing or nailing. This absorber consists ofthe exposed high frequency absorber 12 which may be, for example, hair felt, wool, cotton, excelsior, wood fibers, asbestos, mineral wool, rock wool, sound absorbing plaster, other porous ceramic materials, exfoliated vermiculite and the like. This layer may be from inch to 1 inch thick or any other-desired thickness depending upon the nature of the sounds to be ab- 3 sorbed. Back of this high frequency absorber is an absorbing construction which efficiently absorbs the lower frequencies. The low frequency construction is preferably made by facing an absorber 13, which may be such as is used in the high frequency absorbing portion, with a septum 14 having openings 15 therein. The absorber is preferably one which in itself is more efficient in the lower part of the high frequency range. The

openings 15 are so spaced and of such area that they in combination withthe absorber 13, provide a construction which has its highest efficiency in the frequency range below that of the high frequency absorber, and usually in the low frequency range below 500 cycles. The two combined constructions give a high absorbing efli-. ciency over a wide range of frequencies. These openings 15 usually are small, holes about 0.075 inch in area being suitable,'an'd spaced apart sufliciently to give the desired low frequency absorption. The holes may vary considerably in size. Such holes spaced from one to three inches apart usually give the desired result though the spacing in some cases may be much greater. The septum 14, which preferably does not transmit 56 sound appreciably without the openings present, may be of paper, wood veneer, metal, composition and ceramic materials, rubber, and the like. It may vary over a wide range of thicknesses,

though preferably it is thin. It may be a sepa-' rate sheet or plate or it may be formed in situ as by spraying or brushing a paint, lacquer, glue, latex, or the like into the adjoining surface of either sound absorber 12 or 13 or both or otherwise densifying the adjoining surface or surfaces and then opening the desired holes in the septum so formed. The holes may be of any desired shape, as for example, round, triangular, square, oblong or oval. They may be long narrow openings which may be made by forming the septum of abutting or adjacent strips of the desired material, or in some cases overlapping strips, of the desired widths. If the composite absorber so formed is made in pieces of uniform size, such as oblong, square or hexagonal blocks or tiles, which are mounted on the ceiling surface in abutting or substantially abutting relation, then the narrow opening or slot between the adjacent tiles acts as an opening in the septum.

The lower frequency portion of the incident sounds must, of course, reach the openings 15 in septum 14, to permit absorption to take place. At the edges of the tiles they may reach the low frequency absorbing construction through the intervening slot or space 16 which acts as the necessary channel from the exposed face of the high frequency absorber 12. If the inter-communicating channels of the high frequency absorber 12 connect with the openings 15 and are of such dimensions and are so interrelated that the lower frequency portion of the incident sound may reach the openings 15 then it usually is not necessary to provide any further channels from the exposed face of the high frequency absorber to these openings. If necessary, special channels 1'7 through the high frequency absorber 12 may be provided, to register with the openings 15, as

shown. These provide an unobstructed path for the low frequency portion of the incident sound. As a result the low frequency sound absorbing material 13 may be of considerably less thickness than would otherwise be the case.

It is also possible to fabricate the duplex construction without any channels such as are shown at 17, and also by omitting the channels formed by the abutting or adjacent sound absorbers at 16. In such cases the sound absorber 12 should be of such character as to permit the low frequency waves to penetrate the same and thus haveaccess to the perforated septum 14 at the rear of the high frequency absorber. The exact construction used is therefore dependent upon the nature of the materials selected, especially for the high frequency absorber.

The high frequency absorber, as stated, may

.be faced with various kinds of materials 18, such as cheesecloth, muslin, felt, the perforated sheet metal or other stiff membrane or sheet of the Norris Patent U. S. 1,726,500, the acoustical fabric of the Weiss Patent U. S. 1,785,507, porous ceramic facings and other types of facings known to the acoustical art. If a perforated facing sheet or perforated plate of low sound absorbing value is used, the perforations should be spaced sufiiciently close together to give the desired high frequency absorption. Wide spacing between openings in the septum 14 causes reflection of high frequency sounds and therefore this wide spacing should not be used between openings or perforations in the facing 18 of the high frequency absorber 12.

Although in Fig. 1, the low frequency absorbing construction contains an absorber 13, it is not always necessary to have this absorber present. Without limiting this application to any definite theory of operation, it is probable that the low frequency absorption depends on a resonance effect. I have found that it is desirable to have a considerable volume available in the region of the perforated septum. That is, if the volume back of the septum containing relatively few perforations is small, the desired absorption is not obtained. One factor of seeming importance is the distance between the septum 14 and the surface back of it, such, for example, as the ceiling 10 in Fig. 1. A resonance chamber of this character I have designated by the term space-volume". This space-volume between the septum and the adjacent surface, in conjunction with the spaced openings in the septum, provides a low frequency absorbing construction. The absorber 13 in said space-volume seems to broaden the low frequency absorbing range and therefore is more desirable for most situations.

The general arrangement shown in Fig. 1 may be varied. Both or either of the high frequency and lower frequency absorbers 12 and 13 may be spaced from the septum 14. Also the lower frequency absorber may be spaced from the surface 10. If a perforated facing or member, or other facing having openings there'n, is associated with the exposed face of the high frequency absorber 12, it may be spaced therefrom; The thickness of both absorbers may be varied to suit the sound absorbing conditions which have to be met.

The high and lower frequency layers and septum may be made a unitary structure by means of a suitable construction. examples of which will bedescribed more in detail hereinafter. For example, hair felt, wool, shredded pulp, excelsior, wood fiber, mineral wool, asbestos, and the like may be glued or otherwise fastened to both sides of a perforated stiff or flexible sheet or-member such as heavy paper, chipboard, asbestos paper, composition board, slats flexibly joined, sheet metal, and the like. If the perforated sheet is flexible the absorber may be produced in long strips which may be rolled into convenient bundles. The combination of absorbers 12 and 13 with septum 14 shown in Fig. 1 may be such a pre-assembled construction. The exposed face of the high frequency absorber may also be faced, in advance of installation, with a suitable facing material. If the perforated stiff sheet or membrane facing heretofore identified by reference to said Norris patent is used, the above described unitary material (minus the facing on the high frequency absorber) may be used as the absorber backing for said perforated facing, to thereby provide an absorbing construction highly effective over a wide range of frequencies.-

Tile having 'similar' acoustic properties may be made of those materials now beLngused for acoustic tiles, such as asbestos, mineral wool, hair felt, excelsiqr, wood fiber, bagasse, pumice and other ceramic products. In such tiles, a septum which has spaced openings is used, as previously described in connection with Fig. 1. As a specific example, one side of the structure may be made of a porous mineral wool and sodium silicate mixture onehalf inch thick, weighing eighteen pounds per cubic foot. A similar material may be used for the low frequency side, the two being separated by but adhering to a perforated steel a base for the plaster.

, either or both surfaces of the tile. A porous pumice mixture or other porous material mixture and the like may be used instead of the mineral wool.

My improved absorbing construction also may be used in conjunction with a high frequency absorbing surface of acoustic plaster, one embodi-- ment of which is shown in Fig. 2. The wall or ceiling surface 10 has furring strips or studs 19 mounted on it. On these is nailed or otherwise fastened metal or other lath 20 which serves as Back of the metal lath is a septum 21 which has spaced openings or perforations 22 of such size and number as to give the space-volume back of it lower frequency absorbing characteristics than the acousticplaster. This space-volume preferably contains an absorber 23. The acoustic plaster 24 or similar plastic substance which has the property of absorbinghigh frequency sounds is then applied to the lath. Care must be exercised to keep the openings through the lath open enough to transmit to the perforated septum the low frequency portion of the sound incident to the plastered face, as the space between the two is not in open communication with the room. If desired, apertures 25 communicating with the space between the upper and lower units and hence with the perforations 22, may be formed in the acoustic plaster 24 to insure continuous channels through the plaster. With this construction, a room with a continuous plaster surface has good sound absorption in a wide range of frequencies.

In applying the plaster to the lath in the usual way, it is forced through the openings therein and forms ridges above the same which have the efiect of spacing the perforated septum from the lath, thereby leaving a small clearance space between the two. Such clearance space is therefore incidental to the use of plaster but is not necessarily required forsuccessful sound absortion. Where channels are employed passing through the high frequency absorber 24, said clearance space is probably more desirable, particularly if said channels are not in register with the openings'22 in the septum 21.

In Fig. 3 is shown another variation of the constructionof Fig. 2. Studs or furring strips 19 are mounted on the wall or ceiling surface 10. Wall board 26 or shnilar material having spaced openings 27 is mounted on said studs or furring strips in the ordinary way. The openings are so spaced and are of such size that, in conjunction with the space-volume backing, the desired low frequency absorption is obtained, that is, the wall board is the septum previously described. It is desirable to mount an absorber 28 in this space-volume. An acoustic plaster 29 or any other plastic acoustic materialis applied to the exposed face of the perforated wall board 26. If desired, apertures 30, communicating with perforations 2'7 may be formed in the acoustic plaster 29, to insure continuous channels through the plaster. It is possible also to mount hair felt, ceramic acoustic tile, fiber boards and the like on the wall board in-.

stead of the plaster. If an adhesive is used care must be exercised to prevent such adhesive from filling the perforations.

In Fig. 4 another variation is shown in which the studs or furring strips of Figs. 2 and 3 are omitted although these may be used if desired. The lower frequency absorber 31 in this case is a porous building board usually made of fibrous materials, which has sufficient structural strength to support a plaster coating and which has the porosity necessary to absorb sound with suflicient efiiciency to be used for that purpose. Although the surfaces of some of these boards are dense it usually is desirable to increase this density of one of the surfaces by impregnation with a paint lacquer or the like as shown at 32, said densified surface being then the equivalent, in a certain sense, of the septum in the constructions previously described. Said densified otherwise treated to provide the board with the necessary spaced openings 33 so as to give it the property of absorbing lower frequency sounds. These holes or openings may extend some distance into or through the fiber board as shown, to thereby increase the sound absorption thereof. plastic product, is applied to the exposed surface of the fiber board. It may have apertures 35 through it, as in Figs. 2 and 3. As with the construction shown in Fig. 3, other materials such as hair felt, fiber boards. ceramic acoustic tile and the like may be mounted on the fiber board, care being taken to keep the holes or openings free of adhesive or other impervious material.

Referring to the several forms of the invention described, it is apparent that one aspect of the improvement in general comprises a duplex sound absorber in which the intermediate septum confers low frequency. sound absorbing characteristics to the space-volume back of it, said space-volume optionally containing an absorber. Also,- theimprovement contemplates a duplex construction in which, where sound ,is accessible from all directions, one side may be lined with amembrane containing a few perforations and the opposite side with a membrane containing a larger number of perforations, with or without an intermediate partition or septum.

The above examples are merely by way of illustration. Other forms will be obvious to those skilled in the art.

I claim:

The porous acoustic plaster 34, or similar surface is perforated, grooved, or

1. In combination with a wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of high frequency absorbing construction having a backing of lower frequency absorbing construction 1 in which the space-volume contains a sound absorber.

3. The sound absorbing combination of claim 1 in which the space-volume contains a sound absorber and the septum is of sheet metal.

'4. In combination with a. wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of high frequency absorbing construction of a porous ceramic nature having a backing of lower frequency absorbing construction between it and said surface, said backing comprising a sound absorbing material with a septum between it and said ceramic high frequency absorber, said, septum having openings associated therewith intercommunicating with unobstructed openings through said high frequency absorber and its exposed face to allow the low frequency portion of the incident sound waves to reach said absorber in said lower frequency absorbing construction, said openings being of such area and being spaced sufficiently apart, as described herein, to give, in conjunction with its backing of sound absorbing material, a lower frequency absorption substantially greater than that which would result if said septum were omitted.

5. The sound absorbing combination of. claim 4 in which the ceramic high frequency absorbing layer is a porous sound absorbing plaster.

6. The sound absorbing combination of claim 4 in which the ceramic high frequency absorbing layer is a porous sound absorbing plaster, the absorbing material in the lower frequency absorbing construction is a fibrous material, and the septum is of metal.

7. In combination with a wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of high frequency absorbing construction of a fibrous nature having a backing of lower frequency absorbing construction between it and said surface, said backing comprising a space-volume with a septum between it and said fibrous high frequency absorber, said septum having openings associated therewith inter-communicating with unobstructed openings through which said high frequency absorber and its exposed face to allow the low frequency portion of the incident sound waves to reach said space-volume, said openings being of. such area and such distribution, as described herein, as to give, in conjunction with said spacevolume, an increased low frequency absorption over that which would result if said septum were omitted.

8. The sound absorbing combination of claim '7 in which the space-volume contains a fibrous sound absorber.

9. The sound absorbing combination of claim 7 in which the fibrous high frequency absorber is faced with a material having openings therein of such area and such distribution as to substantially maintain the high frequency absorption thereof.

10. The sound absorbing combination of claim 7 in which the fibrous high frequency absorber is faced with a stiff perforated self sustaining facing material, as described herein, which does not decrease substantially the high frequency absorption thereof and which in itself does not transmit sound appreciably, and in which the space-volume contains a fibrous sound absorber.

11. A sound absorbing construction comprising two adjacent layers of sound absorbing material, the first of said layers being inherently capable of absorbing sounds of high frequency and the second layer having such sound absorbing characteristics as to require a thickness of absorbing material substantially greater than the combined layers in the absence of a perforated septum between them to effect substantial absorption of low frequency sounds, and means comprising a septum interposed between said layers with openings in said septum spaced apart sufliciently to impart to said second layer substantial low frequency sound absorbing characteristics when mounted upon a wall, ceiling surface or other impervious backing with said second absorbing layer between said surface and said perforated septum, said first layer having openings therein communicating with the openings in said septum.'

12. In combination with a wall or ceiling surface and the like, a compound'sound absorbing means comprising two juxtaposed layers of sound absorbing material, the inner layer adjacent said surface being thinner than necessary to effect substantial absorption of low frequency sound if exposed thereto in combination with the outer layer in the absence of a perforated septum between them, the outer layer being capable of effecting substantial absorption of high frequency sounds, and a septum between 'said layers with openings therein spaced apart sufiiciently to impart to said inner layer a low frequency absorption substantially greater than that which would result if said septum were omitted, said openings in said septum communicating with unobstructed openings through said high frequency absorber and its exposed face whereby sounds of a .wide range from low to high frequency vibrations are absorbed by said compound absorbing means.

13. A sound absorbing construction, comprising two adjacent layers of sound absorbing material, one of said layers comprising loose fibrous material from approximately one-half an inch to one inch in thickness, whereby said material may absorb sounds of high frequency, as defined herein, the other of said layers being no thicker than said first layer and composed of similar material, an interposed septum having small openings therein spaced from one to three inchesapart for imparting low frequency sound absorbing characteristics, as defined herein, to said other layer, said high frequency sound absorbing layer having openings therethrough intercommunicating with said septum openings.

14. In combination with a wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of porous material from one-half to one inch in thickness, whereby said material may absorb sounds of high frequency, as defined herein, a backing of lower frequency sound absorbing construction between it and said surface, comprising a space-volume with a septum facing provided with passages through the same, said passages being spaced about one to three inches apart and of much smaller dimensions than said spacing, for imparting low frequency sound absorbing characteristics to said spaceavolume, said high frequency sound absorbing layer having openings therethrough intercommunicating with said septum openings.

15. In combination with a wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of material having a degree of porosity sufllcient to allow sounds of low frequency to pass therethrough, and a backing between it and said surface, comprising a space-volume with a septum facing provided with passages through the same. said passages intercommunicating with the interstices in said material to allow the lower frequency portion of the incident sound waves to reach said space-volume, said passages being of such area and being spaced sufficiently apart as described herein, to give, in conjunction with said spacevolume, a low frequency absorption substantially greater than that which would result if said septum were omitted.

16. In combination with a wall or ceiling surface and the like, a compound sound absorbing means comprising an outer facing of porous material from one-half to one inch irrthickness, whereby said material may absorb sounds of high frequency, as defined herein, without offering substantial resistance to the passage of lower frequency sounds, a backing of lower frequency sound absorbing construction between it and said surface, comprising a, space-volume with a septum facing provided with small openings therein, said openings being spaced apart, as described herein, a distance considerably greater than the diameters of said openings, for imparting low frequency sound absorbing characteristics to said space volume, said high frequency sound CERTIFICATE OF CORRECTION.

Patent No. 1,994,439. March 12, 1935.

KEMPER SLIDELL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 4, first column, line 46, claim 7, strike out the word "which"; and that the said Letters Patent should he read with this correction therein that the same may conform to the record of thecase in the Patent Office.

Signed and sealed this 14th day of May, A. D. 1935.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

